An exhaust gas purifying apparatus has become important, in which exhaust gas from an engine is processed by a catalyst or a filter so that emission of harmful components is suppressed in view of environmental protection. An exhaust gas purifying apparatus is known in the art, for example, in which a diesel particulate filter (DPF) is provided in an exhaust pipe of an engine for trapping the particulates emitted from the engine. The DPF is re-generated by periodically burning out accumulated diesel particulates, based on accumulated amount of the diesel particulates which can be presumed from a differential pressure at the DPF.
In such an exhaust gas purifying apparatus, an over-heat of the DPF has become a problem. The over-heat of the DPF may result from a rapid combustion of the accumulated diesel particulates in the DPF, and may cause a problem in that a breakage of the DPF or a deterioration of catalyst supported by the DPF may occur due to a rapid increase of the temperature of the DPF. The over-heat of the DPF may likely occur when temperature of the exhaust gas flowing into the DPF is high during a high load operation of the engine, or when the temperature of the DPF is extremely increased by a temperature increase operation of the DPF for its re-generation operation. As shown in FIG. 6, it is known that the combustion speed of the diesel particulates is exponentially increased, as the temperature of the DPF becomes higher. Accordingly, it is necessary to suppress the temperature of the DPF at a value lower than a predetermined temperature (e.g. 600° C.), in order to avoid the over-heat of the DPF.
It is, however, not possible to directly detect temperatures at an inside of the DPF by a temperature sensor. Temperatures at a front side and a rear side of the DPF can be detected, when temperature sensors are provided at the front and rear sides of the DPF. However, heat generation is taking place in the inside of the DPF due to the combustion of the diesel particulates and unburned hydrocarbons (HC) contained in the exhaust gas, and furthermore, a large time delay is present between the heat generation at the inside of the DPF and a temperature change caused by the heat generation and appeared at the rear side of the DPF, due to a heat capacity of the DPF (see FIG. 3A). It is, therefore, necessary to presume, with a high accuracy, the temperature at the inside of the DPF, since the temperature detected by the temperature sensor can not be simply regarded as the temperature of the DPF, as explained above.
One of methods for presuming the DPF temperature is disclosed in Japanese Patent Publication No. 2003-254038, in which the DPF is treated in a lumped parameter system and heat budget (heat transfer with the exhaust gas, heat generated amount of the diesel particulates, heat generated amount of the hydrocarbons, etc.) of the total DPF is calculated to presume the temperature.
In the DPF made of ceramics as its basic material, it has a characteristic that heat conductivity is low and a heat spot, in which temperature of a part of the DPF is locally increased, is likely to occur. In the case that the temperature of the center portion of the DPF is rapidly increased due to the combustion of the diesel particulates accumulated in such portion, a large temperature difference appears at a time point A of FIG. 3A. In this case, the temperatures of the other portions than the center portion are lower than that of the center portion, as indicated by a solid line in FIG. 3B. The locally increased temperatures are, however, averaged with the other lower temperatures, and therefore, it is not possible to accurately presume a temperature of the DPF by the conventional method, in which the DPF is treated in the lumped parameter system, as shown in FIG. 3B.